Change stale PR cron schedule to run bi-monthly

Author: bact

.github/workflows/stale.yml

@@ -3,7 +3,7 @@ name: Mark stale pull requests
 
 on:
   schedule:
-  - cron: "0 0 * * *"
+  - cron: "0 0 1,15 * *"
 
 permissions:
   pull-requests: write